Lenses made of resin have rapidly come into wide use as lens resins, for example, spectacle lenses, camera lenses, pickup lenses used in optical recording and regenerating devices, and the like, in replacement of glass lenses, from the viewpoints of light weight, impact resistance, colorability and the like. For this purpose, products of radical polymerization of diethylene glycol bis(allyl carbonate) (hereinafter, abbreviated to D.A.C.), polycarbonate (PC), polymethyl methacrylate (PMMA) and the like have been conventionally used heretofore. However, because these resins for lenses have refractive indexes nd of about 1.49 to 1.58, in order to obtain optical properties equivalent to those of glass lenses, it is necessary to increase the center thickness, edge thickness and curvature of a lens. Thus, the lens becomes very thick in overall. Therefore, there has been a demand for a resin having a higher refractive index.
As a solution for such problems, Patent Document 1 describes a resin obtained by thermal polymerization of a thiol compound and an isocyanate compound to form a thio-urethane bond, and this resin has a refractive index nd of about 1.60 to 1.67.
Furthermore, Patent Document 2 describes a resin obtained by thermal ring-opening polymerization of a thioepoxy compound to form an epithiosulfide bond, and this resin has a refractive index nd of 1.70 or higher.
Meanwhile, although the lenses made of resins have advantages as described above, they are highly inferior to glass lenses in surface hardness and scratch resistance. Thus, as one of measures to handle the problem, it has been essential to perform a hard coating treatment to prevent any scratch from forming on the surface of a lens. As a method of such hard coating treatments, many methods of curing a composition including a polyfunctional (meth)acrylate monomer or oligomer and a radical photopolymerization initiator with ultraviolet ray have been investigated, and various hard coating agents for PC and PMMA have been developed and are widely used for industrial purposes.
However, there still remain the following problems.
[1] A hard coating agent including a polyfunctional (meth)acrylate monomer or oligomer having a functionality of 3 or higher as the main component, has good hard coatability because a cured film formed from the hard coating agent has a high crosslinking density. However, the film formed from the hard coating agent undergoes large shrinkage after curing, and has high residual strain or residual stress. Thus, the hard coating agent may not have sufficient adhesion depending on the resin on which the hard coating agent is applied. Particularly, in the case of the above-described thiourethane resin and thioepoxy resin, it is known to be difficult to obtain sufficient adhesion.
[2] From the viewpoint of the recent attempts to reduce the burden on the environment as much as possible, in order to reduce the use of volatile solvents and to enable the recovery and recycling of liquids to reduce waste materials, there will be a demand in the future for a hard coating agent which does not contain any diluting solvents. But, since polyfunctional (meth)acrylate monomers and oligomers having a functionality of 3 or higher generally have high viscosities, diluting solvents are needed. To decrease the viscosity without using any diluting solvent, a method of using a monofunctional (meth)acrylate monomer having a functionality of 2 or lower as a reactive diluent can be used. However, the hard coating agent using the monomer often does not undergo complete curing, since the agent is likely to be inhibited from polymerizing by oxygen. Even though curing occurs, because the crosslinking density decreases almost proportionally to the amount of the monofunctional monomer, there is a problem that high hard coatability cannot be obtained.
Thus, a method of performing cationic polymerization induced by light is being investigated. Cationic photopolymerization is advantageous over radical photopolymerization, in the points of not being affected by the inhibition of polymerization by oxygen, and having relatively excellent adhesion due to the small shrinkage of the produced film during curing when an epoxy compound or an oxetane compound is used as the monomer. However, under the current circumstances, sufficient hard coatability cannot be obtained.
In addition, Patent Document 3 describes a composition including a cationic photopolymerizable silsesquioxane compound (a), a photocationic compound (b) other than (a), and a cationic photopolymerization initiator (c). Specific examples of the photocationic compound (b) include ethyl vinyl ether and the like as those having a vinyloxy group; bisphenol F diglycidyl ether and the like as those having an epoxy group; and 1,4-bis[(3-ethyl-3-oxetanylmethoxy)methyl]benzene and the like as those having an oxetanyl group. Such a composition tends to have a decreased curing rate, when the proportion of the cationic photopolymerizable silsesquioxane (a) is increased to impart hard coatability. Thus, the composition cannot have sufficient hard coatability, and cannot have adhesion to thiourethane resins or thioepoxy resins. Moreover, the cationic photopolymerizable silsesquioxane (a) generally has a high viscosity even in a liquid phase, and consequently, the composition has an increased viscosity, resulting in deteriorating smoothness as a film.
Even if a hard coating agent which addresses the above-described problems could be obtained, the thiourethane resins and thioepoxy resins used as the substrate are in general known to have poor weather resistance. Thus, the substrates themselves may undergo cohesive failure under exposure to sunlight or rain, and as a result, there still remains a problem of deterioration in the adhesion of the hardcoat formed from the hard coating agent.
Patent Document 1: Japanese Laid-open patent publication No. 9-110956
Patent Document 2: Japanese Laid-open patent publication No. 2002-194083
Patent Document 3: Japanese Laid-open patent publication No. 11-116682